In the past, butterfly valves have been constructed on the offset disc or vane principle. In this type of construction, the centerline of the seal seat mating surface is offset from the actuating shaft centerline. With this construction, 360 degrees of sealing could be effected without shaft interference. However, several problems are inherent with the offset seal construction. One is the longer face-to-face requirement. Another is that only unidirectional sealing can be effected.